Bob,

our atm is around 21% O2.  19.5% is considered oxygen deficient.  CO is formed from incomplete combustion.   Internal combustion engines run at much lower O2 concentrations (torr not %.  Besides CO breaks down naturally in the presence of O2. Our total atmospheric O2 will never drop to a low enough level barring nuclear war or asteroid impact for that to happen.  There is not enough fossil or organic carbon out there to use up all the O2.

Yes, there is lots of oxygen out there. We are in
no danger of running out! I should have put my
post in persepective.
 But it is interesting that the depletion of
oxygen in the atmosphere, mostly from combustion
process, is measureable.